The invention relates to semiconductor devices such as transistors and the method of making the same.
Semiconductors such as transistors are normally manufactured with the active semiconductor elements contained on a header of metallic material and with the electrodes protruding from the header thru insulators. A metal case produced by a stamping operation is positioned over the header to protect the header and active elements of the semiconductor from damage. When the metal case is formed by the stamping operation, minute particles of metal cling to the finished stamping despite extreme care and costly efforts to move and/or detect the particles. Among the measures taken, the stamped metal case is thoroughly cleaned and the case and transistor are thoroughly inspected.
The particles from the case are so small that they cannot be detected by normal methods. They are on the order of one to two one-hundredths of a microgram in mass and are very thin. A particle 0.001 inch to 0.002 inch in diameter would be considered a very large particle. The shape of the particle prevents it from concentrating with others in one position. The particle is so small that it makes no noise if the transistor is shaken and the particle is bounced against the case.
Attempts have been made to screen out transistors with these undesired particles but such methods as acoustical detection under vibration have been unsuccessful. The small metal particles or flakes which cling to the metal case remain attached to the case until the right condition arises during handling when a particular type of shock or a particular vibration will cause the particles to detach from the case with the result that they short the active elements on the semiconductor chip which is positioned atop the transistor header. This causes failure of the transistor in the system which it is controlling with an accompanying loss of time and expense while a technican trouble shoots the system. This has been a significant and costly problem. A solution to this problem has been sought which would provide a highly effective seal while maintaining commercial practicality.